【作者】 田雨；

【导师】 纪卓尚；

【作者基本信息】 大连理工大学 ， 船舶与海洋结构物设计制造， 2011， 博士

【摘要】 目前为止,重大损伤(碰撞与触礁等)对极限强度的影响已经进行了大量的研究,但是近年来海难事故的频繁发生,使得另一种损伤现象_一低周疲劳及其损伤对极限强度的影响引起船舶业界人士的注意。与碰撞与触礁现象不同,疲劳损伤的影响不是瞬时的,而是随时间增长不断累积的,和腐蚀一样,疲劳是影响强度的重要时间因素之一。传统的疲劳强度评估中的疲劳载荷基于设计波法,循环应力范围较小,由此引发的高周疲劳损伤较小,因而疲劳损伤对极限强度的影响通常被忽略。但是研究发现,低周疲劳损伤对船舶结构剩余强度的影响要远大于高周疲劳损伤,因此准确评估船舶在制造以及使用过程中产生的低周疲劳损伤对极限强度的削弱作用是十分重要的。同时,低周疲劳为高应力幅应变疲劳,疲劳循环应力幅接近材料的屈服极限,循环寿命低于104次,因此低周疲劳强度本身也是衡量船舶结构承载能力一个重要的方面。随着现代化船舶设计及建造技术的发展,低周疲劳损伤对剩余强度的影响日益受到研究人员的重视,建立合适的计算模型,分析船舶结构极限承载能力随着疲劳损伤的衰减规律,是进一步进行船舶结构可靠性分析以及风险分析的基础。能够精确地计算船舶结构剩余强度,则可以预报新船在未来运营中的剩余强度变化过程,得到营运过程中老龄船舶结构的真实承载能力,同样也可作为船舶营运时强度监控系统的理论依据。本文目的是综合分析当前含低周疲劳损伤结构剩余强度的研究概况,对含疲劳损伤船舶结构剩余强度的评估方法做一较为深入系统的研究,综合考察低周疲劳损伤缺陷对典型船舶单元结构极限强度的影响,给出带损伤构件剩余强度计算方法,为船体板架以及船体梁的剩余强度评估提供基础。围绕这一目的,本文具体研究内容如下：(1)系统归纳和总结了含疲劳损伤结构剩余强度的研究方法和研究进展；在吸收前人研究结果的基础上,找到合理地解决含低周疲劳损伤船舶结构剩余强度的分析方法：(2)分析船舶结构发生低周疲劳损伤的原因,准确计算统计波浪载荷下低周疲劳载荷,建立低周疲劳强度评估准则,得到合理的疲劳强度分析流程：在理论分析方面,基于连续介质损伤力学原理,从热力学角度给出低周疲劳损伤的力学定义,建立低周疲劳强度及损伤模型,将计算结果与试验结果做一比较,为进一步分析低周疲劳损伤对剩余强度的影响奠定基础；(3)采用虚拟无损单元的张量形式推导含损伤板的剩余强度,得到具有低周疲劳损伤板剩余强度的通用张量表达式；用有限元数值计算方法对含低周疲劳损伤板的剩余强度进行参数化分析,考察由于损伤引起材料特性、结构几何尺寸变化对板的极限强度的影响,在此基础上对张量表达式进行标量简化,得到船舶正交异性无加筋板结构剩余强度经验公式；(4)在平板分析结论的基础上,使用虚拟无损单元对含低周疲劳损伤加筋板单元剩余强度进行参数化有限元数值分析计算,将无加筋平板剩余强度的经验公式推广到加筋板,得到船舶加筋板结构剩余强度经验公式；(5)对现有船舶整体结构剩余强度评估方法进行总结,在评估方法中考虑低周疲劳因素的影响,利用加筋板剩余强度分析结论,评估低周疲劳损伤对船体板架和船体梁结构剩余强度的影响。综上所述,本文基于损伤力学热力学和塑性力学理论,对低周疲劳损伤及其对船舶结构剩余强度的影响进行研究,结合数值分析结果,建立能够合理分析低周疲劳损伤对结构剩余强度影响的评估方法。分析表明,低周疲劳损伤的存在极大削弱了结构的剩余强度,低周疲劳的影响在船舶工程结构强度评估中应该受到足够的重视。同时,船舶结构中损伤分布的数量和分布位置是随机的,本文的损伤剩余强度理论为疲劳随机损伤和风险分析奠定了基础。更多还原

【Abstract】 There have been amount of studies about the effects of crash, aground and other serious damage on ultimate strength, and recently another damage influence, low cycle fatigue, has been brought to forefront by researchers. As the same with corrosion, fatigue is one of the most important time influences over strength. Traditional fatigue assessment method of ship structures is about high cycle fatigue, caused by repeated wave loadings, which damage has less effect on ultimate strength and thus be neglected. However, low cycle fatigue damage will decrease ultimate strength more than high cycle fatigue damages, so it is of crucial importance to estimate the cumulative low cycle fatigue damage effects on residual ultimate strength of aging ship’s structures. Meanwhile, because low cycle fatigue stress amplitude is closed to material’s yield stress and failure cycle number is lower than 104, itself also is an important assessment criterion of ship structures carrying capacity.As the development of modernized skills of design and construction of ships, more attention are paid to estimate the effect of low cycle fatigue damage on residual ultimate strength. For further study of ship structures’reliability and risk assessment, building its assessment models are also foundation to analyze the degradation rules of residual ultimate strength. The degradation models can be used to predict newer ship’s residual ultimate strength, obtain the actual carrying capacity of aging ship structures, and also can be used as theoretical assistant of strength monitoring system in service.So this thesis aims at comprehensively reviewing the study process of residual ultimate strength assessment of low cycle fatigue damaged ship structures, discussing the analysis methods, investigating the effects of damages on the ultimate strength of typical ship structural elements, providing reasonable assessment methods and presenting residual ultimate strength assessment systems of complex ship structures and ship hull girders. In this thesis, to achieve this purpose, efforts and contributions have been made as the following:(1) Systematically summarizing current study methods and progress on residual ultimate strength of damaged ship structures; establishing reasonable assessment systems to analyze the effect of damages on residual ultimate strength;(2) Conducting reasons caused ship structures’low cycle fatigue damages, calculating low cycle fatigue stress amplitude, and establishing failure criterion and assessment procedure of low cycle fatigue; based on thermodynamic theory of continuum damage mechanics, defining damage variables with mechanical definition, building low cycle fatigue strength and damage models, and validating them with experiment results for next analysis of residual ultimate strength;(3) Taking damaged unstiffened plates as fictitious undamaged elements, and deriving general tensor expressions of ultimate strength of plate structures with low cycle fatigue damages; conducting finite elements analysis in order to assess low cycle fatigue damage’s effect on material characters, structural extent and other factors which deteriorate ultimate strength, then obtaining empirical formula of ultimate strength of unstiffened damaged plates by simplifying tensor expressions;(4) On basis of conclusions about residual ultimate strength of unsitffend plate structures, taking damaged stiffened plates as fictitious undamaged elements, and expanding empirical formula of unstiffened plates to stiffened plates; conducting finite element analysis to verifying the empirical formula of ultimate strength of stiffened damaged plates;(5) Summarizing present ship hull girder residual ultimate strength methods, improving direct calculation method considering fatigue damages, and estimating local components’ low cycle fatigue damage effects of on ship hull girder structures.From the foregoing, in this paper based on theories of continuum damage mechanics and plastic mechanics, it is studied the effects of low cycle fatigue damages on residual ultimate strength of aging ship structures. Combined with FE analysis results, assessment methods have been derived and it is indicated that low cycle fatigue damages will deteriorate ultimate strength seriously. Besides, since the damage positions and amounts are random and the deterministic models in this paper can be the foundation of random fatigue damage and risk assessment for next studies.更多还原